1. FIELD OF THE INVENTION
This invention relates in general to anti-skid braking systems for tandem axle vehicles and in particular to an electronic anti-skid circuit which cooperates with an improved vehicle suspension to increase braking efficiency and eliminate brake hop.
2. DESCRIPTION OF THE PRIOR ART
During braking, a truck or other heavy duty vehicle is subject to severe static and dynamic forces which decrease braking efficiency and make it more difficult to maintain the vehicle under control. One example of this detrimental behavoir occurs when such a vehicle with a conventional suspension system, running unloaded or lightly loaded, is braked under sudden or emergency conditions. The forces generated subject the vehicle to severe vibrations known as "brake hop" or "wheel hop". These vibrations are characterized by vertical oscillations of the rear axle or rear set of axles and their associated wheels towards and away from the road. The resultant impacts of the wheels on the road produce forces which impair the safety of the operator and the equipment by reducing the operating life of the vehicle, shifting the load thereby damaging the cargo and the equipment, and generally reducing the operator control of the vehicle.
Many trucks and heavy duty vehicles have tandem axles which employ two spaced independently sprung axles having equalizing connections to equalize the loads on the axles. Common tandem axle construction utilizes springs which are rigidly secured to the axle seats by a suitable clamp or fastener device. Although braking forces generate brake hop to some degree in both axles, the brake hop is experienced primarily in the front axle of the pair which renders the equalizer connections ineffective during braking so that the vehicle weight is supported chiefly by the rear axle.
For example, when the brakes are applied, forces are introduced into the suspension which tend to rotate the axles about respective control points. These forces are transferred through the springs which are fixedly attached to the axles as a force couple or moments creating a downward force at the forward ends of the springs and an upward force at the rear ends of the springs. Reacting forces on the springs come into play and the result is that the wheels on the axles, and especially those on the front axle, rise from the road and subsequently return thereto. The cycle of axle and wheel rise and fall repeats during brake application with increasing frequency, shaking the vehicle, leading to early breakdown of the same and even endangering the vehicle driver who is subjected to great strain by the impacts as well as the stress of maintaining the vehicle under control.
U.S. Pat. No. 3,074,738, issued on Jan. 22, 1963 to James C. Ward and assigned to the assignee of the present application, discloses an improved suspension unit which includes an equalizing linkage interposed between the axle and spring and the vehicle chassis. This linkage includes an equalizer beam, rockably connected with the chassis and employed against a spring end, and a torque arm, pivotally connected at one end with the axle seat and at its other end with the equalizer beam in such a manner as to cancel the torsional forces created during a brake application. With vehicles having tandem running gear, the rockable equalizer beam is positioned between the axles and the respective axle torque arms are engageable therewith so as to overcome the torsionable forces. Therefore, the rigidly mounted springs are relieved of the braking torque so that they may perform their intended function of supporting vehicle load properly. Furthermore, the torque arms extending from the axle seats are connected to a common reference, namely the equalizer beam, to maintain thereby a common relation between the axles which thus may be maintained substantially parallel at all times.
Another example of detrimental behavior during braking is skidding due to wheel lock-up under severe or emergency conditions. Anti-skid control systems have been developed which modulate the brakes at one or more wheels, alternately applying the brakes to slow the vehicle and releasing the brakes to prevent lock-up of the wheels.
Typical anti-skid systems include three basic components, which are a wheel mounted tone wheel (also known as a rotor or an excitor), a sensor which picks up a signal from the tone wheel rotating with the vehicle wheel, and a control circuit or skid computer which utilizes the information from one or more sensors to direct the application or release of the brakes. In lighter vehicles, such as automobiles and light trucks, a single skid computer connected to separate sensors and tone wheels for each of the wheels may be utilized with satisfactory results. Generally, the front wheels of an automobile are more heavily loaded than the rear wheels thereof and this difference in loading is accenuated during braking conditions. Since the less heavily loaded rear wheels have a greater tendency to lock-up, some anti-skid systems compare the speed of rotation of the rear wheels with the speed of rotation of the front wheels and release the rear brakes when the difference is greater than a predetermined amount. Other systems utilize a standard reference signal to compare with the speed of rotation of each wheel where all the wheels are controlled separately or to compare with the lower speed of rotation of the two wheels at either end of the vehicle.
One form of anti-skid system for larger trucks and heavy duty vehicles has utilized the standard reference signal with braking control on a per axle basis, the comparison being with the lower of the speeds of rotation of the wheels attached to the axle. A select low circuit selects the wheel having the lower rotational velocity and compares the rate of change of that velocity, which is proportional to deceleration, to a standard reference signal representative of a skid condition. When the value of the rate of change equals or exceeds the reference signal the brakes associated with the wheels on the axle are modulated to eliminate the skid condition. If the vehicle is equipped with one or more tandem axles, where two axles are subjected to similar loading, the anti-skid system may sense a skid condition and control the brakes on a per side basis as disclosed in U.S. Pat. No. 3,847,446 issued on Nov. 12, 1974 to Ronald S. Scharlack.